An example of line scanner collection geometry is depicted in FIG. 1A. As shown, an imaging system 10 moves across a surface to be imaged, for example, the imager may be an air-borne or space-borne imager above the Earth. A scanning mechanism scans the limited field of view (FOV) of the imaging system across the surface, transverse to the direction of motion of the imaging system. As the imaging system moves forward in the along track direction 12, the scanner continues to sweep the FOV across the surface as indicated by scan lines 13 in the cross track direction. The combination of scan motion and instrument motion allows the imaging system to observe and measure a region around the ground track that is much larger than the imager's FOV. The scan lines may be pieced together in order to obtain an extensive image of the surface.
A conventional detector array of a line scanner is shown in FIG. 1B. As shown, the scanner includes a 2×6 detector array having 12 detector elements. Each detector element is designated as 15. The 12 detector elements are arranged in a rectangle forming 6 rows and 2 columns. The platform containing the line scanner is moving in the along track direction, while the FOV is scanned in the cross track direction.
A conventional line scan imaging system architecture is depicted in FIG. 2. The imaging system 10 includes a scanner 21, an imaging optics 22, a detector array 23, an electronics module 24 and a data processing module 25. The scanner 21 sweeps the FOV of the imaging optics 22 from side to side. The detector array 23, at the focal plane of the imaging optics, generates an electrical signal, for each detector in the array, that is proportional to the scene radiance. The electronics module 24 converts the detector signals into digital counts, which may then be processed by the on-board data processing module 25. The data may then be transmitted to a ground system, generally designated as 29, via a communications link, designated as 28, where the data may be subjected to additional processing by another data processor system 26. After final processing, an image 27 may be formed for display on a monitor or printer, or may be stored in memory for later retrieval by a viewer.
There are many different scanner architectures that have been used by line scanners. A simple and compact (for a given entrance pupil size) line scanner is a one mirror single-axis scanner, as shown in FIG. 3. The exemplified scanner 30 includes a mirror 31 mounted on a rotating shaft 32. The rotating shaft, or scanner shaft 32 is collinear with both the nominal velocity vector of the vehicle and a telescope optical axis, the latter designated as 33. The angle between the scanner shaft 32 and a normal to the surface of mirror 31 is 45°. The scanner shaft rotates at a constant rate forming a rotation angle about the scanner shaft (which is defined later as θscan). The scanner also scans the FOV 34 of the optics of the telescope across the surface of the Earth in the cross track direction. While scanning in the cross track direction, the scanner moves through nadir and a maximum scanning angle, the latter formed at the an end of scan (EOS). Generally, the Earth surface data is only collected for scan angles within 56° of nadir, for reasons that will be explained later.
Conventional image scanners suffer from three major shortcomings, namely, (a) unwanted image rotation during scan due to the angles of incidence on the mirror's surface changing for the off-axis optical rays as the scanner shaft rotates, (b) a large resulting footprint (which is the projection of the instantaneous FOV of a detector onto the ground), and a corresponding lowered spatial resolution with increasing scan angle, and (c) a ground sample distance (GSD) getting larger with increasing scan angle. These are explained below.
The first shortcoming is due to a single mirror scanner geometry causing the image to rotate on its focal plane. The image rotation relative to a fixed detector array is equivalent to rotation of the detector array relative to a fixed Earth. Some conventional systems, like VIIRS, use an additional moving mirror to remove this rotation. The additional moving mirror adds mass, complexity, and moving parts to the system with more potential for failure. Other systems, like AVHRR and GOES, accept the rotation as a required fact. The AVHRR system only has one detector per channel and, thus, minimizes the effect of the image rotation. The GOES system, however, has 2 or 8 detectors per channel and must consider the errors caused by the rotation. A saving grace for the GOES system is that the rotation is only a few degrees across the scan axis. In general, however, any rotation of the image greatly complicates the use of linear detector arrays.
The quality of the imagery collected by a line scanner is also affected by the collection geometry, as shown in FIG. 4. The geometry is shown as a function of range to the center of the Earth. Both air-borne and space-borne image resolution suffers, because the range to the Earth changes with scan angle. As may be seen in FIG. 4, both range to the surface of the Earth (R) and the LOS zenith angle (θz) to the ground surface increase with scan angle θ in the cross track direction. The magnitude of the effect depends on the altitude h of the imager above the Earth. It is much worse for a space-borne imager, since h and R become much larger than similar parameters for an aircraft-borne imager.
The problem of footprint growth may be best illustrated by mapping the instantaneous FOV (IFOV) of an AVHRR system onto the ground of the Earth at both nadir and at end-of-scan (EOS) of the scanning mirror. This is shown in FIG. 5. As shown, the collection geometry distorts the IFOV, so that a 1×1 km square IFOV at nadir becomes, approximately, an 8×3 km rhombus at EOS. Since spatial resolution depends strongly on the IFOV, the resulting spatial resolution is significantly degraded at EOS. For the AVHRR system, the spatial resolution becomes so poor at scan angles larger than 56° that the data would be of little use to any users and so is not even collected. Most users likely prefer to have the same high resolution at EOS that they can obtain at nadir.
An additional shortcoming of conventional systems is the non-uniform ground sample distance (GSD) between nadir and EOS. In order to reduce design complexity, most line scanners use a constant scan rate and a constant detector sample rate. These constant rates result in a varying distance between samples on the ground. The GSD of the AVHRR system, for example, grows from nadir to EOS in proportion to the IFOV growth from nadir to the EOS. Most users, in contrast, prefer to have images that are sampled at equal intervals on the ground and, thus, result in equal GSDs.
As will be explained, the present invention overcomes the aforementioned shortcomings, by producing a system that collects data at a constant footprint, a constant GSD, and a constant spatial resolution. The present invention achieves all of this when scanning the Earth, or any other extended object.